Color center fluorescence and spin manipulation in single crystal, pyramidal diamond tips

At a Glance

Metadata	Details
Publication Date	2016-11-07
Journal	Applied Physics Letters
Authors	Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonuşen, N. Savenko
Institutions	Tallinn University of Technology, Saarland University
Citations	22
Analysis	Full AI Review Included

6CCVD Technical Documentation: High-Coherence Single-Crystal Pyramidal Diamond Tips for Quantum Magnetometry

This document analyzes a key publication demonstrating the viability of commercial, MPCVD-grown single-crystal diamond (SCD) tips for advanced quantum sensing applications, specifically leveraging Nitrogen-Vacancy (NV) and Silicon-Vacancy (SiV) color centers. The findings validate the need for high-purity, custom-doped SCD material, a core specialty of 6CCVD.

Executive Summary

The analyzed research confirms that commercially available, single-crystal diamond (SCD) pyramidal tips—fabricated via Microwave Plasma Enhanced Chemical Vapor Deposition (MPCVD)—are highly effective platforms for quantum magnetometry.

Material Validation: SCD pyramidal tips were proven to be single-crystal, confirmed by the presence of four distinct pairs of NV ODMR resonances, making them superior for high-coherence sensing compared to polycrystalline or bulk-attached nanodiamonds.
High Coherence Achieved: NV electronic spins demonstrated a crucial coherence time (T${2}$) of 7.7(3) µs, significantly higher than typical high-pressure high-temperature (HPHT) diamond (T${2}$ ≈ 1 µs).
Sensitivity Performance: The resulting quantum sensors achieved calculated AC magnetic field sensitivity ($\eta_{AC}$) of 22.9 nT/$\sqrt{Hz}$, establishing the devices as ready-to-use magnetic field probes.
Photonic Enhancement: The pyramidal geometry actively enhances photoluminescence (PL) collection efficiency for color centers positioned near the apex, driving estimated photon count rates exceeding 500 Mcps.
Process Tuning Demonstrated: Reactive Ion Etching (RIE) using Ar/O$_{2}$ plasma was successfully applied to optimize the NV/SiV ratio and remove highly silicon-doped material formed during initial growth, illustrating a critical post-processing step for device refinement.
6CCVD Value Proposition: The entire fabrication process relies on the supply of custom-doped, high-quality SCD substrates, a specialized capability offered globally by 6CCVD.

Technical Specifications

The following table summarizes the key quantitative performance metrics and physical parameters extracted from the experiment, demonstrating the efficacy of the MPCVD diamond tips.

Parameter	Value	Unit	Context
Coherence Time (T$_{2}$, Average)	7.7(3)	µs	NV centers, base of pyramid
Coherence Time (T$_{2}$, Apex)	7.2(1)	µs	NV centers, apex of pyramid
T$_{1}$ Relaxation Time	0.24(7)	µs	NV centers
AC Magnetic Field Sensitivity ($\eta_{AC}$)	22.9	nT/$\sqrt{Hz}$	Calculated sensitivity
DC Magnetic Field Sensitivity ($\eta_{DC}$)	130.6	nT/$\sqrt{Hz}$	Calculated sensitivity
Rabi Frequency	3.76	MHz	Coherent spin manipulation
ODMR Contrast (C)	7.4	%	Optically Detected Magnetic Resonance
Maximum Si Concentration	Up to 10¹⁹	cm^-3	Confined near the apex due to substrate etching
Pyramid Height	10	µm	Before post-processing
Apex Radius of Curvature	2 - 20	nm	Suitable for high-resolution AFM/sensing
Estimated PL Count Rate	> 500	Mcps	Estimated into the first lens
CVD Nitrogen Volume Fraction	0.1	%	Added to gas mixture for NV center creation

Key Methodologies

The pyramidal diamond tips utilized in this quantum sensing research rely on precise MPCVD growth and specific post-growth processing steps.

Substrate Preparation: Silicon (Si) substrates were pretreated with micron-sized diamond powder to enhance nucleation density, promoting bottom-up growth of the pyramid structures.
MPCVD Growth (Bottom-Up):
- Process: Microwave Plasma Enhanced Chemical Vapor Deposition (MPCVD).
- Gas Mixture: CH${4}$/H${2}$ base mixture.
- Doping (Nitrogen): N was added to the mixture at a volume fraction of 0.1% throughout the process to ensure NV center formation at all heights.
- Structure Formation: The process parameterization ensured that (100) facets grew slowest, resulting in pyramidal SCD micro-crystals embedded in defective, nano-crystalline material.
Post-Growth Annealing: Heat treatment in air was applied to entirely remove the highly-defective nano-crystalline material, allowing the transfer and mounting of the SCD pyramids.
Device Integration: Pyramids were mounted one-by-one onto AFM cantilevers using epoxy.
Performance Optimization (RIE):
- Purpose: To tune the NV/SiV ratio and improve coherence by removing highly Si-doped diamond near the apex.
- Method: Inductively-coupled Reactive Ion Etching (RIE) plasma.
- Recipe: Ar/O$_{2}$ (50 sccm each), 18.9 mTorr, 500 W ICP, 200 W RF power.
- Result: Approximately 1 µm of diamond removed, significantly reducing SiV signal while preserving NV coherence (T$_{2, etched}$ = 8.9 µs).

6CCVD Solutions & Capabilities

This research highlights a critical need for high-quality, customized diamond material suitable for complex fabrication protocols involving precise doping, etching, and integration. 6CCVD is uniquely positioned to supply the foundational materials necessary to replicate, optimize, and scale this quantum sensing technology.

Applicable Materials

The successful replication or extension of this research requires extremely high-purity, defect-engineered SCD substrates.

Optical Grade Single Crystal Diamond (SCD): Required to achieve the high T$_{2}$ coherence times (7.7 µs) necessary for quantum sensing. 6CCVD offers SCD materials with extremely low strain and low background impurity levels, crucial for stable NV center performance.
Custom Doping: 6CCVD specializes in introducing specific dopants during MPCVD growth:
- Nitrogen (N): Essential for generating the NV centers used for magnetometry. We offer precise, controlled nitrogen incorporation profiles.
- Silicon (Si): While the paper focused on mitigating Si contamination from the substrate, controlled Si doping is required for optimizing SiV centers (used for quantum communication and alternative sensing). 6CCVD can provide material with tailored [N] and [Si] concentrations.
Custom Substrates: We supply robust SCD wafers up to 500 µm in thickness, providing the ideal starting material for engineers developing optimized pyramidal tip fabrication processes (either bottom-up or advanced top-down approaches).

Customization Potential

The experimental findings clearly indicate that the final device performance relies heavily on precise material engineering and microfabrication capabilities.

Research Requirement	6CCVD Custom Capability	Engineering Advantage
Specific Tip Geometry (10 µm height)	SCD Plates up to 500 µm thickness	Provides flexible depth for fabricating complex structures like arrays or thicker tips.
Advanced Etching (RIE)	Custom Laser Cutting/Dicing Services	Pre-dicing of wafers facilitates integration and simplifies subsequent etching, mounting, or RIE processing steps.
Future Integrated Devices	Custom Metalization Capabilities (Au, Pt, Pd, Ti, W, Cu)	For integrated quantum circuits (as mentioned in Ref. 21), 6CCVD offers in-house metallization services, enabling immediate deposition of ohmic or sensing contacts (e.g., microwave strip lines for ODMR).
High Surface Quality	Ultra-Polishing (Ra < 1 nm SCD)	Ensures minimal surface defects that can degrade T$_{2}$ coherence, supporting high-fidelity sensing devices.
Logistics	Global Shipping (DDU/DDP)	Ensures rapid, reliable delivery of sensitive materials to research facilities worldwide.

Engineering Support

The challenges faced in this research—such as managing Si concentration and optimizing post-growth RIE plasma recipes—require deep expertise in CVD kinetics and defect physics.

6CCVD’s in-house PhD material scientists and technical engineers are available to consult on complex material requirements for NV/SiV-based Quantum Magnetometry and scanning probe applications. We assist clients in designing specific doping recipes to control color center density, location, and coherence properties, thereby accelerating research and development cycles.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We investigate bright fluorescence of nitrogen (NV)- and silicon-vacancy color centers in pyramidal, single crystal diamond tips, which are commercially available as atomic force microscope probes. We coherently manipulate NV electronic spin ensembles with T2 = 7.7(3) μs. Color center lifetimes in different tip heights indicate effective refractive index effects and quenching. Using numerical simulations, we verify enhanced photon rates from emitters close to the pyramid apex rendering them promising as scanning probe sensors.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4967189